Read the following passage carefully. Underline, highlight and/or circle what
you think to be important parts of the passage. Then answer the questions about
the passage (circle the answer to each question).

Earth's North Magnetic Pole hasn't always
been in the northern hemisphere. Earth's magnetic field reverses at irregular intervals! Sometimes several million years pass between reversals. At other times, only a few thousand years go by between reversals. Earth's magnetic field
has reversed 17 times in the last 4 million years. A magnetic reversal doesn't happen overnight. A reversal occurs gradually over the course of a few thousand years. During a reversal, Earth's magnetic field gradually weakens, eventually dropping almost to zero. The magnetic field intensity then slowly builds back up, but with the locations of the poles switched.

Many rocks record the orientation of the Earth's magnetic field at the time
they are formed. It's like a little record of the magnetic field got frozen
into that rock. So, by looking at ancient rocks, we can tell the polarity of
the magnetic field of the Earth even back to ancient times!

In the 1950's, magnetometer instruments started to be towed by ships traversing
the ocean. The magnetometer would make a magnetic survey of the sea floor.
Magnetic signatures were more intense than those found on land and they were
arranged in bands parallel to mid-ocean ridges. In fact, areas around mid-ocean
ridges showed that the magnetic signatures in rocks on one side of the ridge
were mirrored by magnetic signatures on the other side of the ridge. So survey
results might look something like this (with N denoting northward orientation
and S denoting southward orientation):

N-S-N-N-S-N-S-S-ocean ridge-S-S-N-S-N-N-S-N

It was deduced that mid-ocean ridges are places where new ocean crust is
being created by upwelling material from the deep interior of the Earth. Another
way to say this is that mid-ocean ridges are places of seafloor spreading.
Here molten rock rises up from the depth of the Earth and makes new crust.
The new crust forms in bands on either side of the mid-ocean ridge, with the
newly created band pushing the already-existing crust a little farther away
from the mid-ocean ridge. And of course, as each new band forms, the current
orientation of the Earth's magnetic field is frozen into it.

This passage is a Windows to the Universe Original.

Questions:

1) The magnetic north pole of the Earth

is always in the northern hemisphere

is always in the southern hemisphere

alternates between the northern and southern hemisphere

comes out from the equator of the Earth

2) According to the passage, which of these is a valid magnetic signature for a mid-ocean ridge region?

N-S-ocean ridge-N-S

S-N-ocean ridge-N-S

N-N-ocean ridge-S-S

S-N-ocean ridge-S-N

3) Mid-ocean ridges are NOT:

places where old crust sinks into the Earth

places where new crust forms

places where magnetic surveys can be taken

areas of sea floor spreading

Sketch the Seafloor Magnetism Pattern of the Model

Now, go to the sea floor
model that your teacher has set up in the classroom. Use your magnetometer
to perform
a magnetic
survey
of
the
sea
floor
model.
Draw the line representing the mid-ocean ridge. Draw an upward facing arrow
when your magnetometer reads north and a downward facing arrow when your magnetometer
reads south. Draw these arrows in order on either side of the mid-ocean ridge.
Label your arrows "north" or "south".
Sketch: